1. Field of the Invention
The present invention relates to a method and apparatus for applying spray urethane to form a skin layer of a part in a mold.
2. Background Art
Solid urethane elastomer material is used as part of a process for forming parts in a mold. The solid urethane elastomer is a two-part composition that is mixed at the spray head before applying the material to a mold. The urethane elastomer is formulated to set-up relatively quickly. For example, the urethane elastomer is sprayed from the spray head as a liquid and begins to set-up in about 10 to 15 seconds after it contacts the mold. In about two minutes, the urethane elastomer is cohesive and its surface is tack-free. Even with this relatively rapid set-up, when the urethane elastomer is sprayed on a vertical surface, the elastomer forms drips or runs on the vertical surface and may accumulate in liquid pools where a vertical surface meets a horizontal surface. It is also difficult to obtain a surface coating having relatively uniform thickness if the urethane elastomer is sprayed on a vertical surface because gravity causes the material to flow. On a horizontal surface, the urethane elastomer is easily applied to form a uniform thickness after it contacts the horizontal surface.
Examples of automotive parts that may include a urethane elastomer skin include instrument panels, inner door panels, consoles, glove box doors, steering wheel covers, body pillar covers, and the like. An example of a process for forming automotive parts having a urethane elastomer skin is provided below. In an in-mold painting process utilizing an aromatic urethane, the process would begin by applying paint to a mold release agent in the mold for the part. After painting, the aromatic or aliphatic urethane elastomer is sprayed onto the paint layer to form a wall having a thickness of approximately {fraction (1/16)} to ⅛ inch. However, if the part has surfaces that are not substantially horizontal, but are vertical or sloped, the urethane elastomer will form drips and pools and have areas of reduced thickness. After the urethane elastomer has solidified, an injection molded part may be inserted into the mold on top of the urethane elastomer skin. The injection molded part provides reinforcement ribs, frames for installation of components such as air bags or instruments, and fastener receptacles. Foam is then injected into the mold to fill the mold and secure the injected molded part to the urethane elastomer skin. The part may then be removed from the mold and trimmed.
Recent developments in the manufacture of vehicles include incorporation of a plurality of air bags in the vehicle interior. Parts such as inner trim panels for doors, instrument panels and steering wheel covers that may be formed by the above-described process must be formed with more uniform wall thickness to meet part specifications for air bag covers.
There is a need for a method and apparatus for forming complexly shaped parts with a solid urethane elastomer skin having a relatively uniform wall thickness. There is also a need for a method and apparatus for making parts with a solid urethane elastomer skin that eliminates drips, runs and pools of urethane elastomer material on the part while assuring adequate coverage of a relatively uniform wall thickness on the part. By eliminating waste and reducing the amount of material required to form the urethane elastomer skin on a part, considerable savings can be achieved.